Method and device for detecting anode effects of an electrolytic cell for aluminum production

ABSTRACT

Process for early detection of an anode effect in an aluminum production cell based on molten salt electrolysis. The cell comprises at least one anode, at least one cathode and cathode connecting conductors and anode connecting conductors. The process comprises:
         measurement of a first electrical voltage signal U 1  between a first cathode measurement point on a cathode connecting conductor and a first anode measurement point on an anode connecting conductor;   measurement of at least one second electrical voltage signal U 2  between a second cathode measurement point on a cathode connecting conductor and a second anode measurement point on an anode connecting conductor, at least one of these second measurement points being distinct from the first measurement points;   determination of the value of at least one signal comparison function F over a determined time period T;   determination of the value of at least one risk indicator A identifying the risk of occurrence of an anode effect, starting from the comparison function.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 National Stage Application ofInternational Application No. PCT/FR02/04163 filed Dec. 4, 2002 whichclaims priority to French Application No. 01/15871 filed Dec. 7, 2001.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to cells for aluminium production by electrolysisof alumina dissolved in an electrolyte based on molten cryolite,particularly using the Hall-Héroult process. It relates moreparticularly to a device and a method for detecting anode effects.

2. State of the Art

Metal aluminium is produced industrially by fused bath electrolysis,namely electrolysis of alumina in solution in a molten cryolite bathcalled an electrolyte bath, according to the well-known Hall-Héroultprocess. The electrolyte bath is contained in pots called “electrolysispots” comprising a steel shell that is lined with refractory and/orinsulating materials on the inside, and a cathode assembly positioned atthe bottom of the pot. Anodes are partially immersed in the electrolytebath. The expression “electrolytic cell” normally denotes the assemblycomprising an electrolysis pot and one or more anodes.

The electrolytic current that circulates in the electrolyte bath and thepad of liquid aluminium through anodes and cathode elements, producesaluminium reduction reactions and also maintains the electrolyte bath ata temperature of the order of 950° C. by the Joule effect. Theelectrolytic cell is regularly supplied with alumina so as to compensatefor the consumption of alumina produced by electrolysis reactions.

One essential factor for achieving uniform operation of an aluminiumproduction pot by electrolysis of alumina dissolved in a moltenelectrolyte bath based on cryolite is to maintain an appropriate contentof dissolved alumina in this electrolyte and consequently to adaptquantities of alumina introduced into the bath to the consumption ofalumina in the pot.

Excess alumina creates a risk of the bottom of the pot getting cloggedwith undissolved alumina deposits that could transform into hard platesthat could electrically isolate part of the cathode. This phenomenonthen causes the formation of very high horizontal electrical currents inthe metal of the pots that interact with magnetic fields to stir themetal pad and cause instability at the bath-metal interface.

Conversely, a lack of alumina may in particular cause the appearance ofthe “anode effect”, in other words polarisation of an anode with asudden increase in the voltage at the terminals of the cell and therelease of large quantities of gaseous fluorides and carbon fluorides(CF_(x)) that have a high capacity to absorb infrared rays encouragingthe greenhouse effect.

Several regulation processes have been developed to control the aluminafeed.

In industrial processes, it is known that an indirect evaluation ofalumina contents can be used by monitoring an electrical parameterrepresentative of the concentration of alumina in the said electrolyte.This parameter is usually the variation of the resistance R at theterminals of the pot powered at a voltage U, including acounter-electromotive force Ue for example evaluated at 1.65 Volts andthrough which a current I passes such that R=(U−Ue)/I. Typically,processes for regulation of the alumina content consist of modulatingthe alumina feed as a function of the value of R and its variation withtime. Many patents have been made based on this basic principle, untilvery recently (for example see French application FR 2 749 858corresponding to U.S. Pat. No. 6,033,550).

Therefore, these regulation processes provide a means of maintaining thealumina content in the bath within a narrow and small range and thusobtaining current efficiencies of the order of 95% with acid baths, bysimultaneously and significantly reducing the quantity (or frequency) ofanode effects on pots that are counted as the number of anode effectsper pot and per day (AE/pot/day), called the “anode effect rate”. Thisrate is between 0.15 and 0.5 AE/pot/day for the most recent electrolyticcells (that use point feed systems).

The increasingly strict requirements in terms of the emission ofgreenhouse effect gases are encouraging aluminium producers to searchfor means of further reducing anode effect rates.

Therefore the applicant has searched for economic solutions to thesedifficulties that could be applied on an industrial scale.

SUMMARY OF THE INVENTION

An object of this invention is a process for early detection of anodeeffects in an aluminium production cell based on electrolysis in moltensalt, in which a first electrical voltage signal U1 and at least onesecond electrical voltage signal U2 are measured at two distinctlocations in the said cell, and in which the value of at least one riskindicator A identifying the risk of occurrence of an anode effect (or an“anode effect early indicator” A) is determined starting from ananalysis of the said signals U1, U2, . . . , that can provide an earlyindication that there is a high risk of the occurrence of an anodeeffect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section through a typical electrolytic cell usingpre-baked anodes made of a carbonaceous material.

FIG. 2 illustrates a method of measuring the voltage at the terminals ofan electrolytic pot according to the invention.

FIG. 3 diagrammatically illustrates an anode effect early detectiondevice according to the invention.

FIG. 4 diagrammatically illustrates a part of an anode effect earlydetection device according to the invention.

FIGS. 5 and 6 show voltage and current signals measured according to theinvention on an electrolytic cell.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An anode effect early indicator A is typically determined by comparingthe signals U1, U2, . . . More precisely, the indicator A (or indicatorsA1, A2, . . . ) is (are) typically determined from a function F (U1, U2,U3, . . . ), called the comparison function, which is preferablysuitable for quantifying signal spreading and more specificallydifferences E between the signals U1, U2, U3, . . . .

For example, in one simplified variant of the invention, an indicator Amay be given by an algebraic difference between the two electricalvoltages when two voltage signals are measured, or by an algebraicdifference between extreme values (for example between the signals withthe greatest separation) or between at least two signals when more thantwo voltage signals are measured. According to another variant, anindicator A may be determined statistically, for example by a standarddeviation between all signals. It may also be determined by moresophisticated analogue or digital processing.

The indicator(s) A is (are) preferably determined from the variationwith time of the comparison function F (U1, U2, . . . ), typicallystarting from the variation with time of at least one difference Ebetween the signals Ui (for example an algebraic difference, a standarddeviation, etc.). In other words, an anode effect early indicator A maybe given by an indicator B of the variation with time of the comparisonfunction.

The applicant has observed that, surprisingly, a large proportion ofanode effects begin a long time (up to several tens of minutes) beforethe actual occurrence of the anode effect and that this starting pointcorresponds to the beginning of polarization that results in amodification of the distribution of the electrical voltage in the cell,particularly close to the anode that could be polarized. The applicantalso observed that voltage measurements in at least two distinctlocations of an electrolytic cell are capable of reliably detectinginitiation of an anode effect in advance.

Electrical voltage measurements have the advantage of beingcost-effective and that they can be automated.

Another object of the invention is a process for regulating a moltensalt electrolytic cell for the production of aluminium comprising theanode effect early detection process according to the invention.

Another object of the invention is a device for early detection of anodeeffects in an aluminium production cell by electrolysis in molten salt,capable of using the detection process according to the invention,including at least one first means of measuring a first electricalvoltage signal U1 on the said cell, at least one second means ofmeasuring at least one second electrical voltage signal U2 on the saidcell, and at least one means of determining an anode effect indicator Astarting from an analysis of the said electrical voltage signals U1, U2,. . . , typically starting from a comparison between the signals andpossibly starting from a quantification of variations with time of thedifferences between them.

Another object of the invention is an electrolytic cell and a system forregulation of a molten salt electrolytic cell for the production ofaluminium including an anode effect early detection device according tothe invention.

The invention is advantageously applicable to an electrolytic cell (1)for the production of aluminium by electrolytic reduction of aluminadissolved in an electrolytic bath (15) based on cryolite, particularlyusing the Hall-Héroult electrolysis process.

As illustrated in FIG. 1, an electrolytic cell (1) for the production ofaluminium by the Hall-Héroult electrolysis process typically comprises apot (20), at least one anode (13), at least one cathode (5) and aluminafeed means (18). The pot (20) comprises internal sidewalls (3) and iscapable of containing a liquid electrolytic bath (15). The cell (1) cancarry a so-called electrolytic current with an intensity I circulatingin the said bath. The aluminium produced by the said reductionparticularly forms a “liquid metal pad” (16) on the cathode(s) (5). Theanodes (13) are typically supported by the attachment means (11, 12) toan anode frame (10) that may be mobile. The pot (20) normally comprisesa steel shell (2), inner lining elements (3) and cathode elements (5, 6)that include connection bars (or cathode bar) (6) to which electricalconductors (7, 8) are fixed that are used to carry the electrolyticcurrent.

Several electrolytic cells are usually arranged in series. An“electrolytic” current (for which the total intensity is Io) circulatesin the cells and is distributed in them. The electrolytic current passesin the electrolyte bath (15) through the anode(s) (13) and thecathode(s) (5). It passes from one electrolytic cell to the next throughconnecting conductors (7 to 12) and more precisely through cathodeconnecting conductors (6, 7, 8) of one pot called the upstream pot, andanode connecting conductors (9, 10, 11, 12) of the next pot called thedownstream pot.

The purpose of feeding the cell with alumina is to compensate for themore or less continuous consumption of the cell essentially due to thereduction of alumina into metal aluminium. The alumina feed is usuallyregulated independently, and consists of adding alumina into the liquidbath (15). Feed means (18) typically include crust breakers—feeders (19)that bore a hole in the alumina crust (14) and introduce a dose ofalumina in the opening (19 a) formed in the alumina crust by boring.

Aluminium metal (16) produced during the electrolysis normallyaccumulates at the bottom of the pot and a fairly clearly definedinterface is set up between the liquid metal (16) and the bath based onmolten cryolite (15). The position of this bath-metal interface varieswith time; it moves up as liquid metal accumulates at the bottom of thepot and it moves down when liquid metal is extracted from the pot.

In one preferred embodiment of the invention, the anode effect earlydetection process in an aluminium production cell (1) based on moltensalt electrolysis is characterised in that it comprises:

-   -   measurement of a first electrical voltage signal U1 between a        first cathode measurement point (301 to 304) on a cathode        connecting conductor (6, 7, 8) and a first anode measurement        point (311 to 314) on an anode connecting conductor (9, 10, 11,        12);    -   measurement of at least one second electrical voltage signal U2        between a second cathode measurement point (301 to 304) on a        cathode connecting conductor (6, 7, 8) and a second anode        measurement point (311 to 314) on an anode connecting conductor        (9, 10, 11, 12), at least one of these second measurement points        being distinct from the said first measurement points;    -   determination of the value of at least one signal comparison        function F (U1, U2, . . . ) over a determined time period T;    -   determination of the value of at least one risk indicator A        identifying the risk of occurrence of an anode effect, starting        from the said comparison function(s).

The determined time period T, which is a variable parameter for theprocess according to the invention, may be zero or practically zero (forexample it may be equal to a sampling period Te=1/Fe). It has been foundadvantageous to use a sufficiently large period T to eliminate randomfluctuations of the voltages Ui.

It is advantageous to include the measurement of several distinctelectrical voltage signals U1, U2, U3, . . . as illustrated in FIG. 3.In other words, the detection process according to the inventioncomprises the measurement of N electrical voltage signals Ui, where N isadvantageously more than 2. The use of several signals can increase thereliability of early detection and more precisely determine the positionof the area of the pot in which an anode effect may occur. In this way,the anode effect preventive treatment may for example include a localmodification of the alumina feed (typically within the area detected bythe measurements).

In the detection process according to the invention, the said electricalvoltage signals Ui (in other words U1 U2, U3, . . . Un) are usuallymeasured as a function of time. They are typically measuredanalogically, and are then converted into digital signals forprocessing.

The comparison function F (U1, U2, . . . ) may be given by an equivalentfunction F′ (TU1, TU2, . . . ) that uses pre-processed signals (TU1,TU2, . . . ) as arguments, in other words signals TU1, TU2, . . .derived from pre-processing of the signals U1, U2, . . . . Typically,the pre-processing includes sampling of the real signals U1, U2, . . .at a determined frequency Fe, and possibly one (or more) additionalprocessing operations on at least one of the signals. These operationsare typically chosen from among frequency filtering operations(low-pass, band-pass or other), sub-sampling, calculation of at leastone average (such as an RMS (Root Mean Square), possibly sliding, thatcan be calculated using the relation Urms=√(Σ(Ui(j)−Ur)²/m), where Ui(j)is a value of the voltage Ui at time j, Ur is a reference value,possibly zero, and m is the number of terms in the sum; the samerelation may be used for calculating an average TUrms on thepre-processed signals TUi) and known mathematical operations (such asthe calculation of a difference between each signal Ui or pre-processedsignal TUi and a reference value Uo that may be an average Um of thesignals Ui or the pre-processed signals TUi). These operations can becombined. An anti-aliasing low-pass filter is advantageously included inthe pre-processing. The signals may be processed analogically and/ordigitally. Only some signals Ui may also be pre-processed.

There may be several different types of frequency filtration operation.It has been found advantageous to use a low-pass type filter. The filtercut-off frequency is advantageously between 0.001 and 1 Hz.

It has also been found advantageous to use a band-pass type filter. Lowcut-off and high cut-off frequencies of the band-pass type frequencyfilter are advantageously between 0.001 and 1 Hz and between 1 and 10 Hz(typically 0.5 and 5 Hz) respectively.

In one embodiment of this variant, the pre-processing comprises twofrequency filtrations, one of the low-pass type (with a cut-offfrequency typically equal to about 0.5 Hz) that gives a firstpre-processed signal TUi, and the other of the band-pass type (with alow cut-off frequency typically equal to about 0.5 Hz, and a highcut-off frequency typically equal to about 5 Hz) that gives a secondpre-processed signal TUi′. In this embodiment, the process comprises twocomparison functions F, one applicable to TUi signals and the otherapplicable to TUi′ signals.

In another embodiment of this variant, the pre-processing comprisesthree frequency filtrations; a first of the low-pass type (with acut-off frequency typically equal to about 0.003 Hz) that gives a firstpre-processed signal TUi, a second of the band-pass type (with a lowcut-off frequency typically equal to about 0.003 Hz and a high cut-offfrequency typically equal to about 0.5 Hz) that gives a secondpre-processed signal TUi′, and a third of the band-pass type (with a lowcut-off frequency typically equal to about 0.5 Hz and a high cut-offfrequency typically equal to about 5 Hz) that gives a thirdpre-processed signal TUi″. In this embodiment, the process includesthree comparison functions F, the first applicable to TUi signals, thesecond applicable to TUi′ signals, and the third applicable to TUi″signals.

In one advantageous embodiment of the invention, the said at least onecomparison function F(U1, U2, . . . ) (or possibly F′(TU1, TU2, . . . ))is given by a difference E between the said signals (U1, U2, U3, . . . )or between the pre-processed signals (TU1, TU2, . . . ). In particular,the comparison function F(U1, U2, . . . ) may be given by a difference Ebetween at least two voltage signals U1, U2, . . . , or between at leasttwo pre-processed voltage signals TU1, TU2 . . . . The difference E maybe given by an algebraic difference between the signals Ui orpre-processed signals TUi, for example by the largest difference betweenall signals Ui or pre-processed signals TUi (typically the differencebetween the signals with the greatest separation, at a given time, orover a given time period). The difference E may also be given by astandard deviation between the signals Ui or pre-processed signals TUi.

At least one anode effect early indicator A may be equal to a comparisonfunction F(U1, U2, . . . ) or F′(TU1, TU2, . . . ).

The value of at least one indicator A of the risk of occurrence of ananode effect may also be determined from variations with time of thesaid comparison function(s) F or F′. These variations may be given by anindicator B of the variation with time of a comparison function F(U1,U2, . . . ) or F′(TU1, TU2, . . . ). In one simplified variant of thisembodiment, the comparison function F(U1, U2, . . . ) is given by adifference E between at least two voltage signals U1, U2, . . . orbetween at least two pre-processed voltage signals TU1, TU2, . . . , andthe variation indicator B may be proportional to the difference betweenthe value E(t) of a difference E at time t and its value E(t-to) at timet-to, where to is an adjustable parameter.

The indicator A may signal a severe risk of occurrence of an anodeeffect when its value is greater than a given threshold value S.Typically, the process signals this severe risk when the value of adifference E (and more generally E(t)) is more than a given thresholdvalue Se or when the variation of the value of the comparison function For F′ is greater than a given threshold value St.

In one advantageous embodiment of the invention, the detection processalso comprises a test operation that can reveal the susceptibility of anelectrolytic cell to the initiation of an anode effect. This testoperation typically comprises a temporary reduction in the rate of feedof alumina to the cell (corresponding to under-feed of alumina), thisreduction typically being between 20 and 100% of the average feed rate(100% representing a complete stoppage of the alumina feed). Forexample, tests carried out by the applicant have shown that a temporaryreduction in the feed rate of alumina to the cell, or even a temporarystoppage of this feed, can significantly increase the spread of voltagesUi or pre-processed voltages TUi when the cell is in a high risk state,with respect to the occurrence of an anode effect.

The regulation process according to the invention advantageouslycomprises a preventive anode effect treatment operation that caneliminate anode effects that are detected in advance, and that can beactivated when an anode effect has been detected in advance. Thisoperation is normally triggered as a function of the value of thefunction F (or F′), typically when a difference between at least twosignals Ui or between at least two pre-processed signals TUi exceeds agiven threshold Se, or when the variation of this difference with timeexceeds a given threshold St.

The preventive treatment typically comprises a modification to theposition of the anode(s) with respect to the cathode(s), an excess feedof alumina compared with the normal feed rate, or a combination of theseoperations.

The regulation process advantageously takes account of operatingprocedures that could result in disturbed values for the function F (orF′) and therefore for the indicator(s) A, such as anode changes.

In order to enable preventive treatment of an anode effect, the cell (1)advantageously comprises at least one adjustment means such as a mobileanode frame (10) to which the anode(s) (13) is (are) fixed or a means ofcontrolling the alumina feed means (18, 19).

Advantageously, the regulation process also comprises:

-   -   measurement of at least one voltage signal UA on at least one        cell on the upstream and/or downstream side;    -   comparison between the signal(s) UA and the signals U1, U2, . .        . (or the pre-processed signals TU1, TU2, . . . ) so as to        subtract fluctuations (or noise) from neighbouring cells, and        possibly from the entire series of electrolytic cells, from the        signals U1, U2, . . . , or from the pre-processed signals TU1,        TU2.

According to another variant of the invention, the regulation processalso comprises:

-   -   measurement of at least one electrolytic current intensity        signal I;    -   comparison between the signal(s) I and signals U1, U2, . . . (or        pre-processed signals TU1, TU2, . . . ) so as to subtract        fluctuations (or noise) common to all electrolytic cells, from        the signals U1, U2, . . . or from the pre-processed signals TU1,        TU2 . . . .

The intensity I is typically the total intensity Io circulating in thecells. The intensity I of other currents circulating in a series ofelectrolytic cells could also be used, such as the current circulatingin an anode, in a connecting conductor or in a cathode bar.

In particular, this variant of the invention can reduce the“signal/noise” ratio.

According to one preferred embodiment of the invention, the device forearly detection of an anode effect in an aluminium production cell bymolten salt electrolysis is characterised in that it comprises:

-   -   at least one first means (321 to 344) of measuring a first        electrical voltage signal Ul between a first cathode measurement        point (301 to 304) on a cathode connecting conductor (6, 7, 8)        and a first anode measurement point (311 to 314) on an anode        connecting conductor (9, 10, 11, 12);    -   at least one second means (321 to 344) of measuring a second        electrical voltage signal U2 between a second cathode        measurement point (301 to 304) on a cathode connecting conductor        (6, 7, 8) and a second anode measurement point (311 to 314) on        an anode connecting conductor (9, 10, 11, 12), at least one of        these second measurement points being distinct from the said        first measurement points;    -   at least one means (351–354, 40) of determining the value of at        least one signal comparison function F(U1, U2, . . . ) or        F′(TU1, TU2, . . . ) over a determined time period T;    -   at least one means (50) of determining the value of at least one        risk indicator A identifying a risk of occurrence of an anode        effect starting from the function(s) F or F′.

The device may also comprise a means of determining the value of atleast one risk indicator A identifying a risk of occurrence of an anodeeffect starting from variations with time of the said comparisonfunction(s) F or F′.

The measurement means of the electrical voltage signals U1, U2, . . .advantageously comprise electrical conductors (32, 321, 322, 323, 324, .. . , 33, 331, 332, 334, . . . )—typically in the form of wires orcables—with one end connected to a measurement point (30, 301, 302, 303,304, . . . , 31, 311, 312, 313, 314, . . . ) on the cell and the otherend connected to voltage measurement means (34, 341, 342, 343, . . . )such as a voltmeter. The electrical voltage measurement points (30, 301,. . . , 31, 311, . . . ) may be made by any known means such as screwfasteners, notching, etc.

Some voltage measurement means (30, 31, 32, 33, 34, . . . ) may be fixedpermanently on the cell. They are advantageously installed on fixedparts of the cell such as fixed conductors (7, 8, 9, 10) which, inparticular, avoid measurement interruptions and re-installation ofmeasurement means during anode changes.

The said electrical voltage signals U1, U2, U3, . . . are advantageouslymeasured between a collector (8) and a riser (9), preferably in thelower part (9 a) of the said riser (as illustrated in FIG. 2), which inparticular simplifies the wiring (32, 321, 322, . . . , 33, 331, . . . )and facilitates access to measurement points (30, 301, . . . , 31, 311,. . . ).

The signals S (S1, S2, . . . ) generated by measurement means (34, 341,342, . . . ) that are equivalent to voltage signals U1, U2, . . . , aretransmitted to an analyser or a comparator (40) through transmissionmeans (35, 351, 352, 353, 354, . . . ) such as electrical conductors,radio waves, optical means or any other means.

The means (351–354, 40) of evaluating at least one comparison function F(or F′) for comparing the said voltage signals Ui advantageouslycomprise at least one pre-processing means (401–404) for pre-processingat least one of the signals Ui or equivalent signals Si. Thepre-processing means typically comprise at least one frequency filter,and advantageously a low-pass or band-pass filter. The means ofpre-processing may also be a means of sampling the signals U1, U2 at adetermined frequency Fe. In practice, it may also include one or moreelements typically chosen from among analogue/digital converters (ADC),amplifiers (G), frequency filters (low-pass, band-pass or other),sub-samplers, means of calculating an average on a signal (RMS or othertype), means of calculating an average Um of at least one signal Ui orseveral signals Ui, and known mathematical operators (such as means ofsubtracting a reference value Uo and more precisely of calculating adifference between each signal U1, U2, . . . or pre-processed signalTU1, TU2, . . . , and a reference value Uo, where Uo is typically anaverage Um). When the device comprises a low-pass filter, the cut-offfrequency of the low-pass filter is typically between 0.001 and 1 Hz.When the device comprises a band-pass filter, the low and the highcut-off frequencies of the band-pass filter are typically between 0.001and 1 Hz and between 1 and 10 Hz, respectively. The device may alsocomprise a means of determining an average value Um of the signals U1,U2, . . . , or pre-processed signals TU1, TU2, . . . .

The device may also comprise a means (40, 411) of determining adifference E (and more generally E(t)) (such as an algebraic difference,a standard deviation, etc.) between at least two voltage signals U1, U2,. . . or between at least two pre-processed voltage signals TU1, TU2, .. . .

The device may also comprise a means of determining a variation withtime of at least one signal comparison function F(U1, U2, . . . ) or F′(TU1, TU2, . . . ), such as a variation with time of a difference E (andmore precisely E(t)) between at least two voltage signals U1, U2, . . ., or between at least two pre-processed voltage signals TU1, TU2, . . ..

The means of evaluating a function F (or F′) (40, 401, . . . , 404, 411)and of determining an anode effect indicator A (50) may advantageouslybe grouped into a single means, typically using an electronic circuitand/or common data processing means.

Advantageously, the system for regulation of an electrolytic cellaccording the invention also comprises:

-   -   a means of measuring at least one voltage signal UA on at least        one cell on the upstream side and/or the downstream side;    -   a means of comparing the signal(s) UA and the signals U1, U2, .        . . (or the pre-processed signals TU1, TU2, . . . ) so as to        subtract fluctuations (or noise) from neighbouring cells, and        possibly from the entire series of electrolytic cells, from        these signals.

According to another variant of the invention, the regulation systemalso comprises:

-   -   a means of measuring at least one electrolytic current intensity        signal I (typically the total intensity Io circulating in the        cells);    -   a means of comparing the signal(s) I and the signals U1, U2, . .        . (or the pre-processed signals TU1, TU2, . . . ) so as to        subtract the fluctuations (or noise) common to all electrolytic        cells from these signals.

EXAMPLES

Electrical voltage and current measurements were made on an electrolyticpot in which a current with a total intensity of about 500 kA wascirculating. The measures were spread over several weeks. Six voltagesignals Ui were measured at 6 different locations in the pot, betweenanode measurement points and distinct cathode measurement points. Thecurrent circulating in the six distinct anodes was also measured as afunction of time.

FIGS. 5 and 6 show the results obtained during a 24-hour period duringwhich an anode effect (denoted AE) was observed. FIG. 5 corresponds tothe current signals Ii (graph A) and voltage signals Ui (graph B) as afunction of the time t, digitised and pre-processed using a low-passfilter with a cut-off frequency of 0.5 Hz. FIG. 6 corresponds to thesame digitised signals, but pre-processed using a band-pass filter withcut-off frequencies equal to 0.5 Hz and 5 Hz. In both figures, the graphC gives the difference between each filtered voltage signal Ui and theaverage Um of the 6 filtered voltage signals. The letters CA identifythe moment at which an anode was changed.

A progressive increase in signal spreading was observed several tens ofminutes before an anode effect (denoted AE in the figures) (particularlyfor signals filtered in low-pass). One or more anodes started to bepartially polarized, with polarisation areas increasing relativelyslowly.

FIG. 5 shows that spreading of signals filtered in low-pass increasedgradually before the polarization events. In particular, spreadingincreased significantly (from 9 mV to more than 30 mV) starting 90minutes before strong polarization observed after the temporary cut-offof the alumina feed (denoted SA in FIG. 5). Similarly, spreadingincreased significantly (from 7.5 mV to 12 mV) starting 30 minutesbefore the anode effect denoted AE in FIG. 5. The comparison functioncould then be given by the largest difference between two signals Ui-Um.

An increase in signal spreading was also observed during an anode change(denoted CA in FIG. 5). In this case, the increase took placeimmediately (changing quickly from 8.5 mV to 15 mV). These observationscan be used to correct anode effect risk indicators to overcome theseknown disturbances and in particular disturbances related to operationson the pot or to some specific regulation procedures.

FIG. 6 can be helpful for making another diagnostic on the behaviour ofsignals filtered in band-pass. An increase in the spreading was alsoobserved (which increased from 0.2 mV to more than 0.4 mV in this case)in anode effect risk situations.

A combination of this information may also be used to generate syntheticanode effect risk indicators for reliable early detection of anodeeffects and to apply treatments that could avoid these effects.

LIST OF NUMERIC MARKS

(1) electrolytic cell

(2) shell

(3) inner lining (inner sidewall)

(4) inner lining (refractory bricks)

(5) cathode

(6) connecting bar or cathode bar

(7) cathode connecting conductor

(8) cathode connecting conductor (collector)

(9) anode connecting conductor (riser)

(9 a) lower part of a riser

(10) anode frame

(11) support and attachment for an anode (anode stem)

(12) anode support means

(13) anode

(14) alumina cover (or crust)

(15) electrolyte bath

(16) liquid metal pad

(17) solidified bath layer

(18) alumina feed means

(19) crust breaker-feeder

(19 a) opening in the alumina crust

(10) pot

(30) (301) (302) . . . (31) (311) (312) . . . electrical voltagemeasurement points

(32)(321)(322)(323) . . . (33)(331)(332)(333) . . . electrical conductor

(34) (341) (342) (343) . . . electrical voltage measurement means

(35) (351) (352) (353) . . . transmission means

(40, 401, . . . , 404, 411) means of evaluating a comparison function F

(50) means of determining an anode effect indicator A.

1. Process for early detection of an anode effect in an aluminumproduction cell based on molten salt electrolysis, said cell comprisingat least one anode, at least one cathode and cathode connectingconductors and anode connecting conductors, wherein said processcomprises: measurement of a first electrical voltage signal U1 between afirst cathode measurement point on a cathode connecting conductor and afirst anode measurement point on an anode connecting conductor;measurement of at least one second electrical voltage signal U2 betweena second cathode measurement point on a cathode connecting conductor anda second anode measurement point on an anode connecting conductor, atleast one of these second measurement points being distinct from thesaid first measurement points; determination of a value of at least onesignal comparison function F over a determined time period T;determination of a value of at least one risk indicator A identifyingthe risk of occurrence of an anode effect, starting from said comparisonfunction.
 2. Detection process according to claim 1, wherein thefunction F is given by an equivalent function F′ that uses signals, . .. as arguments derived from pre-processing of signals.
 3. Detectionprocess according to claim 2, wherein the pre-processing comprisessampling the electrical voltage signals at a determined frequency Fe. 4.Detection process according to claim 2, wherein the pre-processingcomprises a frequency filtration operation of at least one of saidelectrical voltage signals.
 5. Detection process according to claim 4,wherein the frequency filtration operation is of the low-pass type. 6.Detection process according to claim 5, wherein the cut-off frequency ofthe low-pass type frequency filtration operation is between 0.001 and 1Hz.
 7. Detection process according to claim 4, wherein the frequencyfiltration operation is of band-pass type.
 8. Detection processaccording to claim 7, wherein the low cut-off and high cut-offfrequencies of the band-pass type frequency filtration operation arebetween 0.001 and 1 Hz and between 1 and 10 Hz respectively. 9.Detection process according to claim 2, wherein the pre-processingcomprises at least one sub-sampling.
 10. Detection process according toclaim 2, wherein the pre-processing comprises the calculation of atleast one average of at least one signal Ui.
 11. Detection processaccording to claim 10, wherein the average is an RMS average. 12.Detection process according to claim 2, wherein the pre-processingcomprises the calculation of a difference between each signal Ui orpre-processed signal TUi and a reference value Uo.
 13. Detection processaccording to claim 12, wherein the reference value Uo is an average Umof the signals Ui or the pre-processed signals TUi.
 14. Detectionprocess according to claim 1, wherein the comparison function F is givenby a difference E between at least two voltage signals, or between atleast two preprocessed voltage signals.
 15. Detection process accordingto claim 14, wherein the difference E is given by an algebraicdifference between the signals Ui or pre-processed signals TUi. 16.Detection process according to claim 14, wherein the difference E isgiven by a standard deviation between the signals Ui or thepre-processed signals TUi.
 17. Detection process according to claim 1,wherein at least one indicator A is equal to a comparison function F orF′.
 18. Detection process according to claim 1, wherein at least oneindicator A is given by an indicator B of the variation with time of acomparison function F or F′.
 19. Detection process according to claim18, wherein the comparison function F is given by a difference E betweenat least two voltage signals or between at least two pre-processedvoltage signals, and wherein the variation indicator B is proportionalto the difference between the value E(t) of a difference E at time t andits value E(t−to) at time t−to, where to is an adjustable parameter. 20.Detection process according to claim 17, wherein the indicator A signalsa severe risk of occurrence of an anode effect when a value of saidindicator A is greater than a given threshold value.
 21. Processaccording to claim 1, wherein said process comprises a test operationthat can reveal the susceptibility of an electrolytic cell to initiationof an anode effect.
 22. Process according to claim 21, wherein the testoperation comprises a temporary reduction in the rate of feed of aluminato the cell.
 23. Detection process according to claim 1, wherein saidprocess comprises the measurement of N electrical voltage signals Ui,where N is more than
 2. 24. Process for regulation of an electrolyticcell, wherein said process comprises the anode effect detection processaccording to claim
 1. 25. Regulation process according to claim 24,wherein said process further comprises an anode effect preventivetreatment.
 26. Regulation process according to claim 25, wherein thepreventive treatment comprises an operation selected from the groupconsisting of (i) a modification to the position of an anode withrespect to a cathode, (ii) an excess feed of alumina compared with anormal feed rate, and (iii) a combination of (i) and (ii). 27.Regulation process according to claim 24, wherein said process furthercomprises: measurement of at least one voltage signal UA on at least onecell on an upstream and/or downstream side; comparison between thesignal UA and the electrical voltage signals or the pre-processedsignals so as to subtract fluctuations from neighboring cells, andoptionally from an entire series of electrolytic cells, from theelectrical voltage signals, or from the pre-processed signals. 28.Regulation process according to claim 24, further comprising:measurement of at least one electrolytic current intensity signal I;comparison between the signal I and the electrical voltage signals orpre-processed signals so as to subtract fluctuations common to allelectrolytic cells, from the electrical voltage signals or from thepre-processed signals.
 29. Device for early detection of an anode effectin an aluminum production cell based on electrolysis in molten salt,capable of using the detection process according to claim 1, said cellcomprising at least one anode, at least one cathode and cathodeconnecting conductors, and anode connecting conductors, wherein saiddevice comprises: at least one first means of measuring a firstelectrical voltage signal U1 between a first cathode measurement pointon a cathode connecting conductor and a first anode measurement point onan anode connecting conductor; at least one second means of measuring asecond electrical voltage signal U2 between a second cathode measurementpoint on a cathode connecting conductor and a second anode measurementpoint on an anode connecting conductor, at least one of said secondmeasurement points being distinct from the said first measurementpoints; at least one means of determining the value of at least onesignal comparison function F or F′ over a determined time period T; atleast one means of determining the value of at least one risk indicatoridentifying a risk of occurrence of an anode effect A starting from thefunction F or F′.
 30. Device according to claim 29, wherein the means ofevaluating the value of at least one function F of voltage signalscomprises at least one means of pre-processing at least one of thesignals.
 31. Device according to claim 30, wherein the pre-processingmeans comprises a means of sampling said electrical voltage signals, ata determined frequency Fe.
 32. Device according to claim 30, wherein thepre-processing means comprises a frequency filter.
 33. Device accordingto claim 32, wherein the frequency filter is a low-pass filter. 34.Device according to claim 33, wherein the cut-off frequency of thelow-pass filter is between 0.001 and 1 Hz.
 35. Device according to claim32, wherein the frequency filter is a band-pass filter.
 36. Deviceaccording to claim 35, wherein the low cut-off and high cut-offfrequencies of the band-pass filter are between 0.001 and 1 Hz andbetween 1 and 10 Hz respectively.
 37. Device according to claim 30,wherein the pre-processing means comprises at least one means ofsub-sampling said electrical voltage signals.
 38. Device according toclaim 30, wherein the pre-processing means comprises at least one meansof calculating an average of at least one signal Ui or several signalsUi.
 39. Device according to claim 30, wherein the pre-processing meanscomprises a means of calculating a difference between each electricalvoltage signal, or pre-processed signal and a reference value Uo. 40.Device according to claim 39, wherein said device further comprises ameans of determining an average value Um of said electrical voltagesignals or pre-processed signals.
 41. Device according to claim 29,wherein said device further comprises a means of determining adifference E between at least two voltage signals or between at leasttwo pre-processed voltage signals.
 42. Device according to claim 29,wherein said device comprises a means of determining a variation withtime of at least one signal comparison function F.
 43. Electrolytic cellbased on molten salt for aluminum production, wherein said cellcomprises an anode effect detection device according to claim
 29. 44.System for regulation of an electrolytic cell based on molten salt foraluminum production, wherein said system comprises an anode effect earlydetection device according to claim
 29. 45. Regulation system accordingto claim 44, wherein said system further comprises: a means of measuringat least one voltage signal UA on at least one cell on an upstream sideand/or a downstream side thereof; a means of comparing the signal UA andthe electrical voltage signals or pre-processed signals so as tosubtract fluctuations from neighboring cells, and optionally from anentire series of electrolytic cells, from voltage signals or frompre-processed signals.
 46. Regulation system according to claim 44,wherein said system further comprises: a means of measuring at least oneelectrolytic current intensity signal I; a means of comparing the signalI and the electrical voltage signals or the pre-processed signals so asto subtract fluctuations common to all electrolytic cells from theelectrical voltage signals or the pre-processed signals.